CN111868815B

CN111868815B - Display device

Info

Publication number: CN111868815B
Application number: CN201880091354.8A
Authority: CN
Inventors: 西山隆之
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 2018-03-27
Filing date: 2018-03-27
Publication date: 2022-07-26
Anticipated expiration: 2038-03-27
Also published as: CN111868815A; US20210027709A1; WO2019186723A1; US11521547B2

Abstract

A display device, comprising: a display element (OLED) that emits light by current flow; drive transistor (M) _D1 ) A control circuit that controls a current flowing in the display element (OLED); and a diode-connected transistor (M) _D2 ) Connected to the drive transistor (M) _D1 ) Source side of (D), a drive transistor (M) _D1 ) The back grid of the grid has a constant potential (V) _B1 )。

Description

Display device

Technical Field

The present invention relates to a display device, and more particularly, to an active matrix type display device.

Background

Among electro-optical elements constituting pixels arranged in a matrix, an organic EL element of a current-driven type is known. In recent years, a display device in which a display device is mounted can be increased in size and reduced in thickness, and development of a display device including an organic EL (Electro Luminescence) in a pixel has been actively performed while paying attention to the vividness of a displayed image.

In particular, the following display devices are often provided: an active matrix display device is provided in which a current-driven electro-optical element and a switching element such as a Thin Film Transistor (TFT) are provided in each pixel, and the electro-optical element is controlled for each pixel. By using an active matrix type display device, it is possible to display an image with higher definition than that of a passive type display device.

Here, in the display device of the active matrix type, a connection line formed in a horizontal direction for each row and a data line and a power supply line formed in a vertical direction for each column are provided. Each pixel includes an electro-optical element, a connection transistor, a drive transistor, and a capacitor. Data can be written by applying a voltage to the connection line to turn on the connection transistor and charging a data voltage (data signal) on the data line to the capacitor. The driving transistor is turned on by the data voltage charged in the capacitor, and a current from the power supply line is allowed to flow to the electro-optical element, whereby the pixel can emit light.

Therefore, in an organic EL display device of an active matrix type using organic EL elements, gradation expression of each pixel is realized by controlling a current value flowing in the organic EL element of each pixel by a voltage applied to a driving transistor and emitting light with desired luminance. In addition, when the organic EL display device displays images at low luminance, since it is necessary to reduce the current flowing through each organic EL element, a subthreshold region in which the gate-source voltage of the driving transistor is equal to or lower than a threshold is used.

Documents of the prior art

Patent literature

Patent document 1: japanese patent laid-open publication No. 2014-44316

Disclosure of Invention

Technical problem to be solved by the invention

However, since the subthreshold characteristic of the driving transistor is a region in which the current value abruptly changes due to a change in the gate voltage, and a gate voltage difference indicating one gray level difference may be smaller than a scale value of a data driver supplying the data voltage, it is difficult to perform good gray level expression. In addition, there is a problem that gradation expression for each pixel is affected due to characteristic variation of the driving transistor, and gradation unevenness occurs.

Therefore, an object of the present invention is to provide a display device which can reduce the influence of variations in characteristics of driving transistors and can realize good gradation expression even at low luminance.

Means for solving the problems

In order to solve the above problem, a display device according to the present invention includes: a display element which emits light by current flow; a driving transistor which controls a current flowing through the display element; and a diode-connected transistor connected to a source side of the driving transistor, a back gate of the driving transistor being inputted with a constant potential.

In such a display device, the relationship between the gate voltage and the current value in the sub-threshold characteristic of the driving transistor is adjusted in accordance with the constant potential input to the back gate of the driving transistor, thereby reducing the influence of the characteristic variation of the driving transistor and realizing good gradation expression even at low luminance.

In one embodiment of the present invention, the constant potential is fixed during a period in which the driving transistor is in an on state.

In one embodiment of the present invention, the back gate is electrically connected to a ground wiring.

In one embodiment of the present invention, one end of a capacitor is connected to the back gate, and the other end of the capacitor is connected to a ground potential.

In one embodiment of the present invention, the back gate is electrically connected to a low-level or high-level power supply wiring.

In one embodiment of the present invention, the back gate is electrically connected to an initialization wiring.

In one embodiment of the present invention, the diode-connected transistor is provided between a high-level power supply line and the driving transistor.

In one embodiment of the present invention, the present invention includes: a first transistor whose drain is connected to a high-level power supply wiring and gate is connected to a light emission control line; a second transistor whose source is connected to an anode of the display element and gate is connected to a light emission control line; a reset transistor whose drain is connected to the initialization line and gate is connected to the first scan line; a switching transistor whose source is connected to the data line and gate is connected to the second scan line; a third transistor whose source is connected to the source of the first transistor and whose gate is connected to the second scan line; and a second capacitor, the driving transistor and the diode connection transistor being connected between a source of the first transistor and a drain of the second transistor, a gate of the driving transistor, a drain of the third transistor, a source of the reset transistor, and one end of the second capacitor being connected to a first node, and a source of the diode connection transistor, a drain of the second transistor, the other end of the second capacitor, a drain of the switching transistor, and the back gate being connected to a second node.

In one embodiment of the present invention, the back-gate-side capacitance of the driving transistor is C _BGI Setting the driving grid side capacitance as C _GI Capacitance ratio k ═ C _BGI /C _GI Then, the subthreshold coefficient S obtained by combining the driving transistor and the diode-connected transistor is expressed by a linear function of k.

In one embodiment of the present invention, when the sub-threshold coefficient of the driving transistor or the diode-connected transistor is S, the sub-threshold coefficient is set to be S ₀ Then, the subthreshold coefficient S obtained by combining the driving transistor and the diode-connected transistor has the following relationship: s ═ 2+ k) S ₀ 。

Effects of the invention

According to the present invention, a display device which can reduce the influence of characteristic variations of driving transistors and can realize good gradation expression even at low luminance can be provided.

Drawings

Fig. 1 is a circuit diagram showing one pixel of an organic EL display device in a first embodiment.

Fig. 2 is a circuit diagram showing an organic EL display device according to modifications 1 to 3 of the first embodiment, in which fig. 2 (a) shows modification 1, fig. 2 (b) shows modification 2, and fig. 2 (c) shows modification 3.

Fig. 3 is a circuit diagram showing an organic EL display device according to

modifications

4 and 5 of the first embodiment, in which fig. 3 (a) shows modification 4, and fig. 3 (b) shows modification 5.

Fig. 4 is a circuit diagram showing an organic EL display device in modifications 6 to 9 of the first embodiment, fig. 4 (a) shows modification 6, fig. 4 (b) shows modification 7, fig. 4 (c) shows modification 8, and fig. 4 (d) shows modification 9.

Fig. 5 is a circuit diagram showing an organic EL display device according to comparative example 1 and

modifications

10 and 11 of the first embodiment, in which fig. 5 (a) shows comparative example 1, fig. 5 (b) shows modification 10, and fig. 5 (c) shows modification 11.

Fig. 6 is a circuit diagram showing an organic EL display device according to modifications 12 to 15 of the first embodiment, in which fig. 6 (a) shows modification 12, fig. 6 (b) shows modification 13, fig. 6 (c) shows modification 14, and fig. 6 (d) shows modification 15.

FIG. 7 is a schematic diagram showing the driving transistor M _D1 Diode-connected transistor M _D2 、M _D3 A circuit diagram of various connection relationships of (2).

Fig. 8 is a graph showing the relationship between the capacitance ratio k and the value of the sub-threshold coefficient S.

FIG. 9 shows the driving transistor M _D1 In the relationship between the gate-source voltage Vgs and the current value Id, fig. 9(a) shows a case where k is 0.5, fig. 9 (b) shows a case where k is 1.0, and fig. 9(c) shows a case where k is 1.5.

Fig. 10 is a circuit diagram showing one pixel of the organic EL display device in the second embodiment.

Fig. 11 is a diagram for explaining an external compensation operation according to the second embodiment, in which fig. 11 (a) shows an operation in reading a TFT, and fig. 11 (b) shows an operation in reading an EL element.

Fig. 12 is a diagram for explaining an internal compensation operation of the organic EL display device according to the third embodiment, in which fig. 12 (a) shows a pre-light emission state, fig. 12 (b) shows a reset state, fig. 12 (c) shows data writing and threshold correction, and fig. 12 (d) shows a light emission state.

Fig. 13 is a timing chart of the organic EL display device in the third embodiment.

Detailed Description

< first embodiment >

Hereinafter, embodiments according to the present invention will be described in detail with reference to the drawings. In the present specification and the drawings, components having substantially the same functional configurations are denoted by the same reference numerals, and therefore, redundant description thereof is omitted. Fig. 1 is a circuit diagram showing one pixel of the organic EL display device in this embodiment mode.As shown in fig. 1, the organic EL display device includes a driving transistor M _D1 Diode-connected transistor M _D2 And an organic EL element OLED.

Drive transistor M _D1 The transistor is a transistor for controlling a value of a current flowing by applying a voltage to a gate, and may be formed of, for example, a metal-oxide-semiconductor field-effect transistor (MOSFET). Drive transistor M _D1 Is connected with a diode-connected transistor M _D2 A current source connected to the drain, and a constant potential V input to the back gate _B1 By applying a data voltage V to the gate _in Thereby obtaining a current I _out And (4) flowing. Here, the constant potential V _B1 Denotes the drive transistor M _D1 The on operation period, that is, at least the light emission period is substantially constant, and does not need to be substantially constant over the entire operation period of the organic EL display device. The term "substantially constant" means that the voltage is not changed, and includes a case where a predetermined voltage is continuously applied from the outside or a case where the voltage applied from the outside is maintained. In fig. 1, an n-type channel is shown as the driving transistor M _D1 But may also be a p-type channel.

Here, the driving transistor M _D1 Diode-connected transistor M _D2 The back gate in the iso-transistor means a gate electrode formed on the opposite side of the gate electrode to which the data voltage is input. For example, in the case of a structure in which gate electrodes are formed on and under the semiconductor layer through the gate insulating film, the bottom gate electrode serves as a back gate electrode when a data voltage is input to the top gate electrode, and the top gate electrode serves as a back gate electrode when a data voltage is input to the bottom gate electrode.

Diode-connected transistor M _D2 Is connected with the driving transistor M _D1 The transistor connected in series with the source of (b) can be used, for example, as the driving transistor M _D1 The same MOSFET. Diode-connected transistor M _D2 And the driving transistor M _D1 Is connected to the source of the diode-connected transistor M _D2 Is connected to the organic EL element OLED. In addition, the diode-connected transistor M _D2 Of a grid electrodeAnd a drain short circuit, which is a commonly known configuration as a diode connection of a transistor.

In addition, a diode-connected transistor M _D2 The back gate and the source are shorted. Diode-connected transistor M _D2 The back gate and the source of (1) may not be short-circuited, but the electric field may be prevented from being wound by the short-circuit, thereby improving the saturation of the MOSFET.

The organic EL element OLED is an electro-optical element that emits light by a current flowing therethrough, and is an element constituting one pixel of the organic EL display device. The anode of the organic EL element OLED is connected to the diode-connected transistor M _D2 Of the substrate. Here, only one of the RGB colors constituting one pixel of the organic EL display device is illustrated.

In the organic EL display device of the present embodiment shown in fig. 1, the voltage is inputted to the driving transistor M _D1 Constant potential V of the back gate of (1) _B1 Adjusting the driving transistor M _D1 The relationship between the gate voltage and the current value in the sub-threshold characteristic (2) is that the change in the current value due to the change in the gate voltage is gradual. Thus, the driving transistor M _D1 Is enlarged to increase the current I _out Data voltage V required for changing 1 gray _in The difference of (2) is large, and the gradation can be controlled well within the control range of the voltage value outputted from the data driver. This can reduce the influence of variations in characteristics of the driving transistors, and can realize good gradation expression even at low luminance.

Next, a modification of the first embodiment will be described with reference to fig. 2 to 6. Fig. 2 is a circuit diagram showing an organic EL display device in modifications 1 to 3 of the first embodiment, in which fig. 2 (a) shows modification 1, fig. 2 (b) shows modification 2, and fig. 2 (c) shows modification 3.

Fig. 2 (a) is a circuit diagram showing modification 1 of the first embodiment. As shown in fig. 2 (a), the organic EL display device of the present modification includes a driving transistor M _D1 Diode-connected transistor M _D2 Organic EL element OLED and switching transistor M _S DATA lines DATA, SCAN lines SCAN, high power supply line ELVDD, and low power supply lineThe horizontal power line ELVSS. The present modification is different from the first embodiment shown in fig. 1 in that: diode-connected transistor M _D2 The back gate and the source are not shorted.

Drive transistor M _D1 Has a source connected with a diode-connected transistor M _D2 A drain connected to a high level power line ELVDD and a gate connected to a switching transistor M _S Of the substrate.

In addition, a constant potential V is inputted to the back grid _B1 . Constant potential V input to the back gate from an external circuit _B1 The constant voltage is preferably supplied, for example, if the configuration is such that the ground potential is supplied, since it is not necessary to add a special circuit for realizing the constant power supply, the number of components can be reduced.

Diode-connected transistor M _D2 Is connected to the drive transistor M _D1 Of the source, diode-connected transistor M _D2 Is connected to the organic EL element OLED, and the gate and the drain are short-circuited. The anode of the organic EL element OLED is connected to the diode-connected transistor M _D2 And a cathode is connected to the low level power line ELVSS. Switching transistor M _S Is connected to the drive transistor M _D1 A source connected to the DATA line DATA, and a gate connected to the SCAN line SCAN.

When a turn-on signal is applied to the SCAN line SCAN, the switching transistor M _S Is turned on, the DATA voltage supplied to the DATA line DATA is applied to the driving transistor M _D1 A gate electrode of (2). Thereby, the transistor M is driven _D1 When turned on, a current flows between the high-level power line ELVDD and the low-level power line ELVSS, and the organic EL element OLED emits light at a luminance corresponding to the current value. The current value flowing at this time and the voltage V supplied from the DATA driver to the DATA line DATA _in And (7) corresponding.

In the present modification, the voltage is similarly input to the driving transistor M _D1 Constant potential V of the back gate of (1) _B1 Adjusting the driving transistor M _D1 The relationship between the gate voltage and the current value in the subthreshold characteristic (2) is that the change in the current value due to the change in the gate voltage is gradual. In this way,the influence of characteristic variation of the driving transistor can be reduced, and good gray scale expression can be realized even under low brightness.

Fig. 2 (b) is a circuit diagram showing modification 2 of the first embodiment. The present modification differs from modification 1 in that: drive transistor M _D1 Is not connected to any signal line, and makes a constant potential V _B1 Floating.

Fig. 2 (c) is a circuit diagram showing modification 3 of the first embodiment. The present modification differs from modification 1 in that: capacitor C _b Is connected to the drive transistor M _D1 The back gate of (1). As shown in (C) of FIG. 2, a capacitance C _b One of which is connected to the back gate and the other of which is connected to the ground potential GND. In this modification, the capacitance C is set _b Connected to the back gate, can reduce the tracking of the parasitic capacitance to the source.

Fig. 3 is a circuit diagram showing an organic EL display device according to

modifications

Fig. 3 (a) is a circuit diagram showing modification 4 of the first embodiment. The present modification differs from modification 1 in that: will drive the transistor M _D1 Is connected to the low-level power line ELVSS. In this modification, a constant potential V is inputted to the back gate _B1 The potential is supplied to the low-level power supply line ELVSS. Thereby, the constant potential V is not added _B1 Input to the drive transistor M _D1 The particular circuit of the back gate in (2) is preferably realized by wiring in the pixel, and thus the number of components can be reduced.

Fig. 3 (b) is a circuit diagram showing modification 5 of the first embodiment. The present modification is different from modification 1 in that: will drive the transistor M _D1 Is connected to the high level power line ELVDD. In this modification, a constant potential V is inputted to the back gate _B1 Becomes a potential supplied to the high-level power supply line ELVDD. Thereby, the constant potential V is not added _B1 Input to the drive transistor M _D1 The special circuit of the back gate in (2) is preferably realized by wiring in the pixel, and thus the number of components can be reduced.

Fig. 4 is a circuit diagram showing the organic EL display device in modifications 6 to 9 of the first embodiment, fig. 4 (a) shows modification 6, fig. 4 (b) shows modification 7, fig. 4 (c) shows modification 8, and fig. 4 (d) shows modification 9.

Fig. 4 (a) is a circuit diagram showing modification 6 of the first embodiment. The present modification differs from modification 1 in that: in the driving transistor M _D1 The organic EL element OLED is disposed between the high-level power supply line ELVDD. As shown in (a) of fig. 4, the anode of the organic EL element OLED is connected to the high-level power line ELVDD, and the cathode is connected to the driving transistor M _D1 Of the substrate. In addition, the diode-connected transistor M _D2 Is connected to the low-level power line ELVSS. In this modification as well, the same effects as those of the first embodiment can be obtained.

Fig. 4 (b) is a circuit diagram showing modification 7 of the first embodiment. This modification differs from modification 6 in that: using a p-type channel as the drive transistor M _D1 In the drive transistor M _D1 A diode-connected transistor M is provided between the organic EL element OLED and the transistor _D2 . As shown in fig. 4 (b), the transistor M is driven _D1 Is connected to the diode-connected transistor M _D2 And a drain connected to the low-level power line ELVSS. In addition, a diode-connected transistor M _D2 Is connected to the cathode of the organic EL element OLED. Even if a p-type channel is used as the driving transistor M as in the present modification _D1 The same effects as those of the first embodiment can be obtained.

Fig. 4 (c) is a circuit diagram showing modification example 8 of the first embodiment. This modification differs from modification 7 in that: at the driving transistor M _D1 The organic EL element OLED is disposed between the low-level power line ELVSS. As shown in fig. 4 (c), the transistor M is driven _D1 Is connected to the diode-connected transistor M _D2 Is connected to the organic EL element OLEDOf (2) an anode. In addition, a diode-connected transistor M _D2 Is connected to the high level power line ELVDD. The cathode of the organic EL element OLED is connected to a low-level power supply line ELVSS. In this modification as well, the same effects as those of the first embodiment can be obtained.

Fig. 4 (d) is a circuit diagram showing modification 9 of the first embodiment. The present modification is different from modification 8 in that: using a p-type channel as a diode-connected transistor M _D2 . As shown in (d) of fig. 4, the diode-connected transistor M _D2 Is connected to a high level power line ELVDD, and has a drain connected to a driving transistor M _D1 Of the semiconductor device. Even if a p-type channel is used as the diode-connected transistor M as in the present modification _D2 The same effects as in the first embodiment can also be obtained.

modifications

10 and 11 of the first embodiment, in which fig. 5 (a) shows comparative example 1, fig. 5 (b) shows modification 10, and fig. 5 (c) shows modification 11. In the figure, VDD denotes a high-level-side voltage, VSS denotes a low-level-side voltage, and the organic EL element is not shown.

Here, in the single transistor, the gate-source voltage is Vgs, the threshold voltage is Vth, the back gate-source voltage is Vbs, the current value is Iout, and the back gate side capacitance of the transistor is C _BGI Setting the driving grid side capacitance as C _GI Let the capacitance ratio k be C _BGI /C _GI And sub-threshold coefficient S ₀ Modeled as follows.

(numerical formula 1)

Iout＝βexp(γ(Vgs-Vth+kVbs))

(numerical formula 2)

Fig. 5 (a) is a circuit diagram showing comparative example 1. The present comparative example is different from modification 1 in that the constant potential V is not set _B1 Input to the drive transistor M _D1 The back gate of (2). When the driving transistor M is formed in the pixel by the same process with the same composition _D1 And a diode-connected transistor M _D2 Then, the transistor characteristics of both are sufficiently similar to the extent that they are considered to be the same, and β, γ, and Vth are equal.

In fig. 5 (a), when the transistor M is driven _D1 And a diode-connected transistor M _D2 When the potential of the connection point x of (b) is Vx,

(numerical formula 3)

Iout ℃ ∈ β exp (γ (Vin-Vx-Vth)) ═ β exp (γ (Vx-VSS-Vth))

(number formula 4)

And Vx ═ (Vin + VSS)/2.

After the numerical expression 4 is substituted into the numerical expression 3,

(number type 5)

Iout∝βexp(γ(Vin-VSS-2Vth)/2)

Will drive the transistor M _D1 And a diode-connected transistor M _D2 The subthreshold coefficient S obtained by synthesis is as follows:

(numerical formula 6)

S＝2S ₀ 。

Fig. 5 (b) is a circuit diagram showing a modification 10 of the first embodiment. The present modification is different from comparative example 1 in that a diode-connected transistor M is provided _D2 The low level side voltage VSS is inputted to the driving transistor M _D1 The back gate of (1). In the present modification, the voltage is inputted to the driving transistor M _D1 Constant potential V of the back gate _B1 VSS. When the above modeling and calculation are used, the driving transistor M of the present modification example is used _D1 And a diode-connected transistor M _D2 The subthreshold coefficient S obtained by synthesis is as follows:

(number 7)

S＝(2+k)S ₀ 。

Therefore, by inputting the low-level side voltage VSS to the driving transistor M _D1 The sub-threshold coefficient S can be represented by a linear function of k, and kS is increased as compared with comparative example 1 ₀ 。

Thereby, the driving transistor M is adjusted _D1 Sub-threshold ofThe relationship between the gate voltage and the current value in the characteristic, and the change in the current value due to the change in the gate voltage becomes gentle. Thus, the driving transistor M _D1 Is enlarged to increase the current I _out Data voltage V required for changing 1 gray _in The difference of (2) is large, and the gradation can be controlled well within the control range of the voltage value outputted from the data driver. This can reduce the influence of variations in the characteristics of the driving transistors, and can realize favorable gradation expression even at low luminance.

Fig. 5 (c) is a circuit diagram showing modification 11 of the first embodiment. This modification is different from modification 10 in that two diode-connected transistors M are provided _D2 、M _D3 Connected in series and inputting a low-level side voltage VSS to the drive transistor M _D1 And a diode-connected transistor M _D2 The back gate of (2). Among the plurality of diode-connected transistors, a side close to the driving transistor is referred to as an upstream side, and a side far from the driving transistor is referred to as a downstream side. The driving transistor M of the present modification is constructed using the above-described modeling and calculation _D1 Diode-connected transistor M _D2 、M _D3 The subthreshold coefficient S obtained by synthesis is as follows:

(number type 8)

S＝(3+3k+k ² )S ₀ 。

Therefore, the subthreshold coefficient S can be represented by a quadratic function over k, and is further increased as compared with modification 10. In the present comparative example, since the square term of k appears in the sub-threshold coefficient S, the amount of increase in the sub-threshold coefficient S becomes larger as the value of the capacitance ratio k becomes larger, and is more preferable.

Fig. 6 is a circuit diagram showing an organic EL display device in modifications 12 to 15 of the first embodiment, in which fig. 6 (a) shows modification 12, fig. 6 (b) shows modification 13, fig. 6 (c) shows modification 14, and fig. 6 (d) shows modification 15.

Fig. 6 (a) is a circuit diagram showing modification 12 of the first embodiment. This modification is different from modification 11 in that two diode-connected transistors M are provided _D2 、M _D3 Are connected in series, and drive the transistor M _D1 Is inputted to the driving transistor M _D1 The back gate of (1). The driving transistor M of the present modification example is constructed using the above-described modeling and operation _D1 Diode-connected transistor M _D2 、M _D3 The subthreshold coefficient S obtained by synthesis is as follows:

(number type 9)

S＝3S ₀ 。

Therefore, the subthreshold coefficient S is preferably 3 times that of the case of a single transistor, and is increased as compared with comparative example 1.

Fig. 6 (b) is a circuit diagram showing a modification 13 of the first embodiment. This modification is different from

modifications

11 and 12 in that two diode-connected transistors M are provided _D2 、M _D3 Connected in series and diode-connected to transistor M _D3 Is inputted to the driving transistor M _D1 The back gate of (2). The driving transistor M of the present modification is constructed using the above-described modeling and calculation _D1 Diode-connected transistor M _D2 、M _D3 The subthreshold coefficient S obtained by synthesis is as follows: (number type 10)

S＝(3+2k)S ₀ 。

Therefore, the sub-threshold coefficient S can be represented by a linear function of k, and is increased by 2kS as compared with modification 12 ₀ And is thus preferred.

Fig. 6 (c) is a circuit diagram showing modification 14 of the first embodiment. This modification is different from modifications 11 to 13 in that two diode-connected transistors M are provided _D2 、M _D3 Connected in series and diode-connected to transistor M _D3 To the diode-connected transistor M _D2 The back grid of (2) connecting the driving diode with the transistor M _D2 To the transistor M _D1 The back gate of (1). The driving transistor M of the present modification is constructed using the above-described modeling and calculation _D1 Diode-connected transistor M _D2 、M _D3 The subthreshold coefficient S obtained by synthesis is as follows:

(numerical formula 11)

S＝(3+2k+k2)S ₀ 。

Therefore, the subthreshold coefficient S can be represented by a quadratic function of k, and is increased as compared with modification 13, which is preferable.

Fig. 6 (d) is a circuit diagram showing modification 15 of the first embodiment. This modification is different from modifications 11 to 14 in that two diode-connected transistors M are provided _D2 、M _D3 Connected in series and diode-connected to transistor M _D3 To the diode-connected transistor M _D2 、M _D3 Will drive the transistor M _D1 Is inputted to the driving transistor M _D1 The back gate of (2). The driving transistor M of the present modification is constructed using the above-described modeling and calculation _D1 Diode-connected transistor M _D2 、M _D3 The subthreshold coefficient S obtained by synthesis is as follows:

(number type 12)

S＝(3+k)S ₀ 。

Therefore, the sub-threshold coefficient S can be expressed by a linear function of k, and is increased as compared with modification 12, which is preferable.

In fig. 5 and 6, a diode-connected transistor M is shown _D2 、M _D3 In the example in which two transistors are directly connected, the number of diode-connected transistors connected in multiple stages is not limited, and may be three or more.

Next, the capacitance on the back gate side of the transistor is represented by C, which is described with reference to fig. 7 to 9 _BGI And the capacitance on the driving grid electrode side is set as C _GI And the capacitance ratio is represented by k ═ C _BGI /C _GI K-dependence of the subthreshold coefficient S. FIG. 7 is a schematic diagram showing the driving transistor M _D1 Diode-connected transistor M _D2 、M _D3 A circuit diagram of various connection relationships of (2). In FIG. 7, (i) shows the driving transistor M _D1 Comparative example 2 alone, (ii) is comparative example 1, and (iii) is the driving transistor M _D1 Diode-connected transistor M _D2 、M _D3 Comparative example 3 connected in series. In fig. 7, (iv) is modification 10, (v) is modification 12, and (vi) is modification 13.

Fig. 8 is a graph showing the relationship between the capacitance ratio k and the value of the sub-threshold coefficient S. The abscissa of fig. 8 represents the capacitance ratio k ═ C _BGI /C _GI The ordinate represents S-value magnification showing that the subthreshold coefficient is S ₀ Several times higher than the desired value. The relationship between the capacitance ratio k and the value of the subthreshold coefficient S in the circuits (i) to (vi) shown in fig. 7 is represented by lines shown in (i) to (vi) in the figure.

As shown in FIG. 8, in (i) to (iii), the sub-threshold coefficient S is S, regardless of the value of the capacitance ratio k ₀ 、2S ₀ 、3S ₀ There was no change. On the other hand, in the modification examples 10 and 12 of the modification examples (iv) and (v), since the sub-threshold coefficient S is expressed by a linear expression of k, the sub-threshold coefficient S increases as the capacitance ratio k increases. In particular, in modification example 10 of (iv), in the region where k > 1, the sub-threshold coefficient S is larger than that of comparative example 3 of (iii). Therefore, even if the diode-connected transistor M is not used _D3 On the other hand, it is preferable to reduce the number of transistors as compared with comparative example 3 of (iii) because the subthreshold coefficient S can be increased. In modification 13 of (vi), since the sub-threshold coefficient S is expressed by a quadratic expression of k, the sub-threshold coefficient S is preferably further increased as the capacitance ratio k increases.

FIG. 9 is a view showing the driving transistor M _D1 Fig. 9(a) shows a case where k is 0.5, fig. 9 (b) shows a case where k is 1.0, and fig. 9(c) shows a case where k is 1.5. In fig. 9, (a) to (c) show the gate-source voltage Vgs on the horizontal axis and the current Id on the vertical axis. The characteristics of the circuits (i) to (vi) shown in fig. 7 are represented by lines (i) to (vi) in the graph.

As can be seen from fig. 9(a) to 9(c), the larger the subthreshold coefficient S, the smaller the slope of the line, and the smaller the change in the current value Id with respect to the gate-source voltage Vgs. In addition, it can be seen that the larger the value of the capacitance ratio k, the smaller the slope of the line, and the smaller the change in the current value Id with respect to the gate-source voltage Vgs. In particular, in the case where the subthreshold coefficient S is represented by a linear expression of the capacitance ratio k, the slope of the line decreases, and when represented by a quadratic expression, the slope of the line further decreases.

As shown in fig. 7 to 9, by inputting to the driving transistor M _D1 Constant potential V of the back gate of (1) _B1 Adjusting the driving transistor M _D1 The relationship between the gate voltage and the current value in the sub-threshold characteristic of (2), and the change in the current value due to the change in the gate voltage becomes gentle. Thereby, the transistor M is driven _D1 The subthreshold region of (A) is enlarged to make the current I _out Data voltage V required for changing 1 gray _in The difference of (a) is large, and the gradation control can be performed well within the control range of the voltage value outputted from the data driver. Therefore, the influence of the characteristic variation of the driving transistor can be reduced, and good gradation expression can be realized even at low luminance.

< second embodiment >

Next, a second embodiment of the present invention will be described with reference to the drawings. The description of the configuration overlapping with that of the first embodiment is omitted. Fig. 10 is a circuit diagram showing one pixel of the organic EL display device in this embodiment.

As shown in fig. 10, the organic EL display device of the present embodiment includes a driving transistor M _D1 Diode-connected transistor M _D2 Organic EL element OLED and switching transistor M _S1 And M _S2 The liquid crystal display device includes a capacitor C, a DATA line DATA, SCAN lines SCAN1 and SCAN2, an initialization wiring, a high level power line ELVDD, and a low level power line ELVSS. Drive transistor M _D1 Diode-connected transistor M _D2 The connection relationship between the organic EL element OLED and the organic EL element is the same as in modification 1 of the first embodiment.

Switching transistor M _S1 Has a gate connected to the SCAN line SCAN1, a source connected to the DATA line DATA, and a drain connected to the driving transistor M _D1 A gate electrode of (2). Switching transistor M _S2 Has a gate connected to the SCAN line SCAN2, a source connected to the anode of the organic EL element OLED, and a drain connected to the initialization wiring. One end of the capacitor C is connected to the driving transistor M _D1 And the other end is connected to the anode of the organic EL element OLED. In addition, the driving transistor M _D1 Is connected to the initial gateAnd (7) forming the wiring.

In the present embodiment, the initialization voltage for initializing the wiring is set as the constant potential V _B1 Is applied to the drive transistor M _D1 Thus, the driving transistor M is adjusted _D1 The relationship between the gate voltage and the current value in the subthreshold characteristic (2) is that the change in the current value due to the change in the gate voltage is gradual. Thus, the driving transistor M _D1 The subthreshold region of (A) is enlarged to make the current I _out Data voltage V required for changing 1 gray _in The difference of (a) is large, and the gradation control can be performed well within the control range of the voltage value outputted from the data driver. This can reduce the influence of variations in characteristics of the driving transistors, and can realize good gradation expression even at low luminance.

Next, external compensation according to the present embodiment will be described with reference to fig. 11. Fig. 11 is a diagram for explaining the external compensation operation of the present embodiment, in which fig. 11 (a) shows the operation during TFT reading, and fig. 11 (b) shows the operation during EL element reading.

First, the SCAN line SCAN1 is set to a high potential and the switching transistor M is set to a high potential _S1 On, a transistor reading DATA voltage is applied from the DATA line DATA to the driving transistor M _D1 A gate and a capacitor C. Thereby, the transistor M is driven _D1 And becomes an on state.

Then, the SCAN line SCAN2 is set to a high potential, and the switching transistor M is turned on _S2 Turned on, as shown in (a) of FIG. 11, measurement is made from the high level power line ELVDD through the driving transistor M _D1 Diode-connected transistor M _D2 And a switching transistor pass transistor M _S2 And the current value flowing through the initialization wiring. By this TFT readout operation, the composite drive transistor M can be read _D1 And a diode-connected transistor M _D2 The transistor characteristics are obtained.

Then, the SCAN line SCAN1 is set to high potential and the switch transistor M is set _S1 Is turned on from the DATA line DATA to the driving transistor M _D1 The gate and the capacitor C apply an EL element reading data voltage. Thereby, the driving transistor M is driven _D1 In a cut-off state and stopStopping the current from the high power line ELVDD.

Then, the SCAN line SCAN2 is set to a high potential, and the switching transistor M is turned on _S2 Turned on, as shown in (b) of FIG. 11, measurement is made from the initialization wiring through the switching transistor M _S2 And a current value flowing to the low-level power line ELVSS through the organic EL element OLED. By this EL element readout operation, the characteristics of the organic EL element OLED can be read.

As described above, the organic EL display device of the present embodiment performs the TFT readout operation and the EL element readout operation and performs the external compensation. Thereby, the driving transistor M can be read _D1 And a diode-connected transistor M _D2 The display characteristics are improved by combining the characteristics of the transistor and the characteristics of the organic EL element OLED and adjusting the DATA voltage supplied from the DATA line DATA.

< third embodiment >

Next, a third embodiment of the present invention will be described with reference to the drawings. The description of the configuration overlapping with that of the first embodiment is omitted. Fig. 12 is a diagram for explaining an internal compensation operation of the organic EL display device according to the present embodiment, in which fig. 12 (a) shows a pre-light emission state, fig. 12 (b) shows a reset state, fig. 12 (c) shows data writing and threshold correction, and fig. 12 (d) shows a light emission state. Fig. 13 is a timing chart of the organic EL display device of the present embodiment.

As shown in (a) to (d) of fig. 12, the organic EL display device of the present embodiment includes a driving transistor M _D1 Diode-connected transistor M _D2 Organic EL element OLED and switching transistor M _S Reset transistor M _R And a transistor M _C 、M _E1 、M _E2 The display device includes a capacitor Cst, a DATA line DATA, a SCAN line SCAN (n), a SCAN line SCAN (n-1), a light emission control line em (n), a high level power line ELVDD, and a low level power line ELVSS. The respective connection relationships are shown in the figure.

Transistor M _E1 Has a drain connected to a high level power line ELVDD and a source connected to the driving transistor M _D1 And a gate connected to the emission control line em (n). Transistor M _E1 First transistor equivalent to the present invention。

Transistor M _E2 Is connected to the node y (n), the source is connected to the anode of the organic EL element OLED, and the gate is connected to the emission control line em (n). Transistor M _E2 Corresponding to the second transistor of the invention.

Transistor M _C Is connected to node x (n), and the source is connected to the drive transistor M _D1 And the gate is connected to the scan line scan (n). Transistor M _C Corresponds to the third transistor of the present invention.

Reset transistor M _R Is connected to the initialization line, the source is connected to node x (n), and the gate is connected to the SCAN line SCAN (n-1). Switching transistor M _S Is connected to the DATA line DATA, the drain is connected to the node y (n), and the gate is connected to the scan line scan (n). Capacitor Cst has one end connected to node x (n) and the other end connected to node y (n). In addition, the node Y (n) is connected to the driving transistor M _D1 The back gate of (1).

A driving transistor M is connected to the node X (n) _D1 Gate of (1), transistor M _C Drain electrode of (1), reset transistor M _R And one end of the capacitor Cst, and corresponds to the first node of the present invention. A diode-connected transistor M is connected to the node Y (n) _D2 Source electrode of (1), transistor M _E2 Drain of (1), the other end of the capacitor Cst, and the switching transistor M _S And a driving transistor M _D1 And corresponds to the second node of the present invention. In addition, the capacitor Cst corresponds to a second capacitor of the present invention, the SCAN line SCAN (n-1) corresponds to a first SCAN line of the present invention, and the SCAN line SCAN (n) corresponds to a second SCAN line of the present invention.

First, in the pre-light emission state shown in fig. 12 (a), as shown in fig. 13 (1), an on signal is supplied to em (n), and an off signal is supplied to SCAN (n-1) and SCAN (n). Thus, the switching transistor M _S Reset transistor M _R Transistor M _C In the off state, the node x (n) is at the potential for pre-illumination. At this time, a current passes through the transistor M from the high level power line ELVDD _E1 And a driving transistor M _D1 Diode-connected transistor M _D2 Crystal of Zi-jinBody tube M _E2 The organic EL element OLED is supplied to the low-level power line ELVSS, and the organic EL element OLED is caused to emit light in advance.

Next, in the reset state shown in fig. 12 (b), as shown in fig. 13 (2), an off signal is supplied to em (n), an on signal is supplied to SCAN (n-1), and an off signal is supplied to SCAN (n). Thus, the switching transistor M _S Transistor M _C 、M _E1 、M _E2 In the off state, the node x (n) is initialized to the potential vini (n).

Next, in the data writing and threshold correction shown in fig. 12 (c), as shown in fig. 13 (3), an off signal is supplied to em (n), an off signal is supplied to SCAN (n-1), and an on signal is supplied to SCAN (n). Thus, the reset transistor M _R Transistor M _E1 、M _E2 In the off state, the transistor M is driven _D1 Switching transistor M _S Transistor M _C Is in a conducting state. At this time, the charge charged to the capacitor Cst in the reset state passes through the transistor M _C And a driving transistor M _D1 Diode-connected transistor M _D2 The switching transistor MS flows to the DATA line DATA, and the node x (n) is the sum of the DATA voltage Vdata and the threshold voltage Vth. Here, the threshold voltage Vth is to drive the transistor M _D1 And a diode-connected transistor M _D2 The threshold voltage in the case of one transistor is synthesized and considered.

Next, in the light emission state shown in fig. 12 (d), as shown in fig. 13 (4), an on signal is supplied to em (n), and an off signal is supplied to SCAN (n-1) and SCAN (n). Thus, the reset transistor M _R Transistor M _C Switching transistor M _S In the off state, the transistor M _E1 、M _E2 A driving transistor M _D1 Is in a conducting state. At this time, the node x (n) maintains the sum of the data voltage Vdata and the threshold voltage Vth through the capacitor Cst. Thus, a current passes through the transistor M from the high-level power line ELVDD _E1 A driving transistor M _D1 Diode-connected transistor M _D2 Transistor M _E2 Organic EL element OLED flowing to low level power line ELVSS, and organic EL element OLEAnd D, emitting light.

As described above, in the organic EL display device of the present embodiment, the internal compensation is performed by performing the pre-light emission and the reset, the data writing, and the threshold correction. Thereby, the driving transistor M can be compensated _D1 And a diode-connected transistor M _D2 The resultant transistor characteristics are synthesized to achieve an improvement in display characteristics.

The present invention is not limited to an organic EL display device using organic EL elements, and the display elements used in the present invention are not limited if the display device includes various display elements whose luminance and transmittance are controlled according to current. Examples of the display element for current control include an Organic EL (Electro Luminescence) display including an OLED (Organic Light Emitting Diode) and an EL display QLED (Quantum dot Light Emitting Diode) such as an inorganic EL display including an inorganic Light Emitting Diode.

The embodiments disclosed herein are merely exemplary in all respects, and are not intended to be construed as limiting. Therefore, the technical scope of the present invention is not to be interpreted only by the embodiments described above, but is defined by the description of the claims. The meaning and range of the claims are intended to be equivalent to those of all modifications.

Description of the reference numerals

M _D1 … drive transistor

M _D2 、M _D3 … diode-connected transistor

M _S 、M _S1 、M _S2 … switching transistor

M _E1 、M _E2 、M _C … transistor

SCAN1, SCAN2, SCAN (n), SCAN (n-1) … SCAN lines

High level power line of ELVDD …

ELVSS … low-level power line

DATA … DATA line

V _B1 … constant potential

Claims

1. A display device, comprising:

a display element which emits light by current flow;

a driving transistor which controls a current flowing through the display element; and

a diode-connected transistor connected to a source side of the drive transistor,

the back gate input of the driving transistor has a constant potential,

the back grid is directly electrically connected with an initialization wiring.

2. The display device of claim 1,

the constant potential is fixed during a period in which the driving transistor is turned on.

3. The display device according to claim 1 or 2,

the diode connection transistor is provided between a power supply wiring of a high level and the drive transistor.

4. A display device, comprising:

a display element which emits light by current flow;

a driving transistor which controls a current flowing through the display element;

a diode-connected transistor connected to a source side of the driving transistor; and

a first transistor whose drain is connected to a high-level power supply wiring and gate is connected to a light emission control line;

a second transistor whose source is connected to an anode of the display element and gate is connected to a light emission control line;

a reset transistor whose drain is connected to the initialization line and gate is connected to the first scan line;

a switching transistor whose source is connected to the data line and gate is connected to the second scan line;

a third transistor whose source is connected to the source of the first transistor and gate is connected to the second scan line; and

the second capacitance is set to be a second capacitance,

the back gate input of the driving transistor has a constant potential,

the driving transistor and the diode-connected transistor are connected between the source of the first transistor and the drain of the second transistor,

the first node is connected with the grid electrode of the driving transistor, the drain electrode of the third transistor, the source electrode of the reset transistor and one end of the second capacitor,

the second node is connected with the source electrode of the diode connection transistor, the drain electrode of the second transistor, the other end of the second capacitor, the drain electrode of the switch transistor and the back grid electrode.

5. A display device, comprising:

a display element which emits light by a current flowing;

the back gate input of the driving transistor has a constant potential,

when the back gate side capacitance of the drive transistor is set to C _BGI Setting the driving grid side capacitance as C _GI Capacitance ratio k ═ C _BGI /C _GI When the utility model is used, the water is discharged,

the subthreshold coefficient S obtained by combining the driving transistor and the diode-connected transistor is represented by a linear function of k.

6. The display device of claim 5,

when separate sub-threshold systems for the driving transistor or the diode-connected transistor are usedNumber is S ₀ Then, the subthreshold coefficient S obtained by combining the driving transistor and the diode-connected transistor has the following relationship:

S＝(2+k)S ₀ 。